1. Field of the Invention
The invention relates to a railroad vehicle signaling apparatus and method for railroad vehicle signaling, particularly, a railroad vehicle information signaling apparatus and method employing a switched-mode transmitter and, more particularly, a railroad vehicle information signaling apparatus employing a stepped-square wave transmitter and method for transmitting carrier-coded railcar information to a railroad vehicle.
2. Description of the Art
Railroad vehicles can receive information such as, for example, speed limit information, by inductively sensing electrical signals in the rails. These signals may consist of a preselected carrier frequency which is modulated on and off at a preselected coding rate. The preselected carrier frequency typically is either 60 or 100 Hertz; and the coding rate typically is 75, 120, or 180 cycles per minute (CPM).
The carrier signal can be generated by switching a DC power source such as a 12 VDC battery, on and off, resulting in a square wave carrier which can be rich in odd harmonics with the third harmonic having one-third as much energy as the fundamental, the fifth harmonic having one-fifth as much energy as the fundamental, etc. Modulating the carrier at the predetermined code rate appends sidebands to each of the harmonics, further adding to the noise spectrum. This noise may preclude the use of some of the other electronic equipment which can be applied across the rails, such as highway crossing motion monitors and predictors, and audio frequency overlay track circuits.
One solution to this problem can be to-use a linear amplifier. This allows a clean sine wave to be applied to the rails, thereby eliminating substantially all of the harmonics. However, this approach increases signal generating circuit complexity and, more importantly, power efficiency. What is needed, therefore, is a method and an apparatus for generating the coded-carrier signals which convey information such as, for example, speed limit information, to the cabs of railroad vehicles and which efficiently produce sufficient signal power with reduced low harmonic-frequency spectral "pollution" inherent in standard designs.